This invention relates to a method for measuring surface and near surface mechanical properties of materials. More particularly, the invention relates to an ultrasonic method for measuring the properties of the surface layer of a specimen in which the surface layer has at least one physical property the value of which differs from the same physical property of the subsurface layer of the specimen.
One of the most difficult objectives in the non-destructive testing of materials is measurement of the properties of a thin surface layer, the properties of which differ from the corresponding properties in the subsurface layer or bulk material lying within a specimen. For example, Berry and Mulhearn, Effective Case Depth: Uncertainties in Measurement, Metallography, 1, 373-385 (1969) discuss the sources of uncertainty in the measurement of effective case hardening depths of surface hardening steel by the traditional destructive method of cutting a section through the case hardened steel and measuring the hardness of various points on the cross-section thus exposed using a loaded, pointed probe. Case-hardening depths have also been measured using eddy current and coercive force measurements (see, for example, Metal Progress, August, 1971, page 55); but such measurements involve the use of complicated and expensive equipment and only yield an average value of case hardening depths over the whole specimen tested. Attempts have also been made to measure the depths of surface layers using Rayleigh or surface waves propagated through the surface layer. For example, Jaspy and Saltoun, Use of Ultrasonic Rayleigh Waves for the Measurement of Applied Biaxial Surface Stresses in Aluminum 2024-T351 Alloy, Materials Evaluation 40, 1982 (February, 1982) describes a time-of-flight measurement technique in which a transmitting ultrasonic transducer is coupled to the surface of a specimen via a plastic wedge and causes propagation of Rayleigh waves through the surface layer of the specimen. Two steel wedges are in contact with the surface of the specimen at points spaced from one another and from the plastic wedge, and pick-up transducers are mounted on the upper ends of either of the wedges. The velocity of the Rayleigh waves through the surface layer, and hence information on the surface layer, is determined by the time-of-flight of the Rayleigh waves between the points at which the two steel wedges contact the surface of the specimen. Since this is a time-of-flight measurement method, it only yields an average value for the properties of the surface layer in the distance between the two steel wedges, a distance which in the experimental apparatus needs to be about 11 millimeters, a distance which may encompass considerable variation of the surface layer in some specimens. Moreover, not only does the method require the use of three transducers and appropriate associated electronics, but it is only really well suited to measurements on specimens having a flat or only gently curving surface; in many practical applications, it would be difficult or physically impossible to engage the transmitting transducer, the plastic wedge and the two steel wedges in position on the specimen.
Two other techniques for measurement of properties of surface layers by means of Rayleigh waves are disclosed in Weston-Bartholomew, Use of the Ultrasonic Goniometer to Measure Depth of Case Hardening (The Corner Reflection Method). In the first of these two methods, a transmitting ultrasonic transducer immersed in a water bath is used to transmit an ultrasonic beam, which impinges upon the surface of a specimen also immersed in the water bath. The impingement of the ultrasonic beam upon the surface produces a reflected ultrasonic beam (in the usual sense of a beam lying at the same angle to the perpendicular to the surface of the specimen, but on the opposed side of this perpendicular), and this reflected beam is detected by means of a second transducer immersed in the water bath. Although this method has the advantage that, since only the properties of the specimen at the point of impact of the beam thereon affect the measurements taken, by the use of relatively narrow beams the method can effectively measure the properties of the surface layer at a single point, the method does require the use of two separate transducers. Furthermore, it is normally necessary to vary the angle of incidence (and thus also the angle of the reflected beam) to the surface of the specimen in order to observe the local maximum of intensity of the reflected beam which occurs at the Rayleigh angle of the specimen. In order to achieve the necessary variation in the angles of incidence of reflection at least two of the three components (the two transducers and the specimen itself) must be mounted for rotation about the same axis which must pass through the surface of the specimen at the point at which the measurements are being taken. In view of the relatively small changes in Rayleigh angle which must be measured in practice (of the order of 0.1.degree.), providing such accurate rotation of at least two components while maintaining proper alignment presents formidable practical difficulties.
In the second method disclosed in Weston-Bartholomew, only a single transducer is used, but a reflector block is mounted on the specimen so that an ultrasonic pulse from the transducer will strike the surface of the specimen, thereby generating a reflected beam which strikes a surface of the reflector block lying perpendicular to the surface of the specimen. The impingement of the reflected beam upon the surface of the reflector block produces a second reflected beam travelling in the opposite direction to the incident beam, so that this second reflected beam can be detected by the transducer, which is of course operated in the pulse-echo mode. Although this second method does require only a single transducer and requires that only one of the transducer and specimen be rotatable, thus reducing many of the alignment problems in the first Weston-Bartholomew method, it does have the disadvantage of requiring very careful alignment of the surface of the reflector block perpendicular to the surface of the specimen, and the time taken for alignment with the necessary degree of precision renders this second method impracticable for use in a routine production line industrial situation. More importantly, since the second method requires the presence of a reflector block of some considerable size immediately adjacent, if not actually attached to, the surface of the specimen, this second method cannot be used to investigate small sections of a component of complex form, for example a toothed gear wheel, because in such situations there would simply be no room for the reflector block.
There is thus a need for a method of measuring the properties of a surface layer of a specimen which uses simple apparatus, which is suitable for use in an industrial production line situation, which is nondestructive and which enables properties of small parts of the surface layer to be measured even on specimens having a complex form. This invention seeks to provide such a method.